1. Field of the Invention
The invention relates to a plasma generator for generating plasma by means of microwave discharge. This plasma generator is well suited for use in, for instance, processes of etching and ashing on a semiconductor wafer placed in a processing chamber in which a plasma gas is introduced or of cleaning of the chamber.
2. Description of the Related Art
This prior art plasma generator is described below with reference to FIG. 10. In FIG. 10, a reference numeral 100 represents a rectangular-sectional waveguide, at an opening of one end side of which is mounted a microwave power source (not shown) for introducing a microwave to the interior of this rectangular waveguide.
Reference numerals 102 and 104 represent cylindrical conductors connected vertically in a tube-axial direction to the walls of the respective open tubes of the rectangular waveguide 100, one cylindrical conductor 102 of which has a gas inlet 102a for introducing plasma and the other cylindrical conductor 104 of which has its opening 104a connected to the processing chamber now shown.
A reference numeral 106 represents a cylindrical insulator, which is inserted through the rectangular waveguide 100 vertically in its tube-axial direction and mounted in both of the cylindrical conductors 102 and 104 and also which has its portion positioned in the rectangular waveguide 100 serving as a plasma generating portion 106a. 
A reference numeral 108 represents a movable terminal which can move in the tube-axial direction so that the plasma generating portion 106a of the cylindrical insulator 106 may have a stronger microwave field therein.
In such a plasma generator, when a microwave is introduced into the rectangular waveguide 100 and a plasma gas is introduced through the gas inlet 102a in the cylindrical conductor 102, plasma is generated due to microwave discharge in the internal space of the plasma generating portion 106a of the cylindrical insulator 106.
The plasma thus generated in the cylindrical insulator 106 is introduced to, for example, a semiconductor wafer positioned in a processing chamber connected to the opening 104a so that this wafer may be etched or ashed or that the chamber may be cleaned.
In the prior art plasma generator, however, since a microwave inside the rectangular waveguide 100 travels perpendicular to the surface of the plasma generating portion 106a of the cylindrical insulator 106, its field acts parallel with the plasma. Accordingly, the microwave has a strong reflected wave power.
To guard against this, conventionally, a matching device is typically attached to the rectangular waveguide 100 to utilize its matching action for suppressing the reflected wave power of the microwave (microwave matching), which is, however, not always easy and is time consuming.
The matching is thus difficult and time consuming, thus leading to problems of inefficiencies of, for example, semiconductor wafer processes, in short, various operations utilizing plasma.
In addition, a large and heavy body of the matching device itself makes it inconvenient to handle it and its high price also increases the costs of the apparatus as a whole.
Also, the prior art plasma generator has poor radiation of the heat generated along with the generation of plasma, thus leading to disabled supply of a high microwave power problematically.
That is, since plasma occurs at a portion (plasma generating portion 106a) of the cylindrical insulator 106 that portion is positioned in the rectangular waveguide 100, the heat due to plasma is generated locally. Moreover, thus locally generated heat is construction-wise difficult to radiate to the outside, so that if plasma is generated by supplying a high microwave power, the cylindrical insulator 106 is highly heated locally and may be damaged. For this reason, the prior art has been impossible to supply a high microwave power.
In order to inhibit the cylindrical insulator 106 from being highly heated locally, a cooling fan may possibly be provided for cooling the plasma generating portion 106a, in which case, however, a required large capacity of the cooling fan would enlarge the apparatus as a whole. Further, such a cooling fan, if used in a clean room for manufacturing of semiconductor devices in which the plasma generator is mounted, may disturb the surrounding atmosphere problematically.
Also, although the cylindrical insulator 106 may possibly be of a double-tube construction so that a cooling medium may be distributed therethrough to radiate the heat, the cylindrical insulator 106 is generally made of a relatively brittle glass tube and also exposed to hot plasma and have the atmospheric pressure applied thereon from the outside because of its internal vacuum state, thus being subject to damages due to heat and pressure. For this reason, the cylindrical insulator 106 must be designed in case of damages, in which case in turn the cooling medium may come into the apparatus to thereby spread its leakage throughout the apparatus, so that such a heat radiating construction could not have been employed.
In view of the above, it is an object of the invention to facilitate microwave matching.
Another object of the invention is to facilitate microwave matching without utilizing a matching device to thereby eliminate the necessity of using it, thus making it easy to handle the apparatus as a whole, and also to reduce the costs of the apparatus.
A further another object of the invention is to cool the apparatus reliably with a simple construction.
The other objects, features, and advantages of the invention will become clear from the following description.
A plasma generator according to the invention comprises, as shown in FIG. 1, a rectangular waveguide (2) in which a microwave is introduced, a coaxial tube (4) connected in a T shape to the rectangular waveguide (2) through their respective openings (2b and 42c), a vacuum-sealing window (6) for blocking the openings (2b and 42c) with an insulating material to thereby vacuum-seal the interior of the coaxial tube (4) against the rectangular waveguide (2), and an insulator (16) arranged in the coaxial tube (4) as linked to the vacuum-sealing window (6).
According to the invention, the insulator (16) is provided at a boundary between plasma and a microwave so that the electric field of the microwave may not run parallel with the plasma. Accordingly, the microwave can be absorbed by the plasma more easily to thereby reduce its reflected wave power. Moreover, by adjusting the shape of the insulator (16), the impedance of the total load can be kept roughly constant against large fluctuations in plasma impedance caused by transitional ignition of the plasma, changes in its process, etc., thus eliminating the conventional necessity of difficult microwave matching by use of a matching device in the rectangular waveguide (2), thus making more easy and efficient a variety of operations with plasma. In addition, it is possible to make it more easy and convenient to use the apparatus even with a large and heavy matching device and also reduce the costs of the apparatus.
Preferably the invention has such a double-tube construction that the above-mentioned coaxial tube (4) may consist of an inner conductor (41) and an outer conductor (42) disposed external of the inner conductor (41) radially with a predetermined gap therebetween and also such an annular construction that the above-mentioned insulator (16) may be mounted on the outer periphery of the above-mentioned inner conductor (41).
By this construction as a whole, a microwave power can be efficiently supplied from the insulator toward a plasma generating space (19) of the radially opposing gap between the inner conductor (41) and the outer conductor (42), thus increasing the plasma generation efficiency in that space preferably.
In this case, the above-mentioned inner conductor (41) may be of a solid construction or a hollow construction.
Also, preferably the invention has such a shape that the insulator (16) may shrink in area of its outer diameter surface as it goes on in a direction opposite to the side of the vacuum-sealing window (6).
In such a case, even when the pressure of a gas supplied to the plasma generating space (19) is changed according to a variety of processes of manufacturing a semiconductor device, it is possible to maintain the reflection coefficient of the microwave at a roughly constant value over a wide variation range of that pressure. Accordingly, preferably it is possible to conduct optimal setting accurately and easily over the variety of processes.
In this case, the above-mentioned diameter shape shrinkage includes not only the above-mentioned directional uniform diameter shrinkage but also partial diameter shrinkage and outer-diameter surface shrinkage for the insulator (16).
The plasma generator according to the invention comprises, as shown in FIG. 2, the rectangular waveguide (2) into which a microwave is introduced, a coaxial tube (4) communicatively connected in a T shape to the opening (2b) made in the rectangular waveguide (2), and a sealing means. The coaxial tube (4) comprises the inner conductor (41) which penetrates through the rectangular waveguide (2) at the opening (2b) in a tube-axial direction of the coaxial tube (4) in such a manner that its one end may protrude from the rectangular waveguide (2) and the outer conductor (42) which is disposed roughly in a coaxial manner with the inner conductor (41) outside the one end of the inner conductor (41). The sealing means blocks at the opening (2b) the gap between the rectangular waveguide (2) and the coaxial tube (4) with the vacuum-sealing window (6) and also vacuum-seals the plasma generating space (19) formed between the outer conductor (42) and the inner conductor (41) against the rectangular waveguide (2). Also, the other end of the inner conductor (41) is coupled to the rectangular waveguide (2) both thermally and electrically.
According to the invention, no plasma is generated inside the rectangular waveguide (2), with the plasma generating space (19) being formed inside the coaxial tube (4) connected in communication to the rectangular waveguide (2). Accordingly, heat generated in the plasma generating space (19) is radiated from the inner conductor (41) through the rectangular waveguide (2) thermally coupled to the other end of this inner conductor (41) to the outside and so is not accumulated in the inner conductor (41), thus avoiding damaging the inner conductor (41). Therefore, it is possible to supply a high microwave power without taking into account the effect of accumulation of heat in the apparatus.
According to the invention, preferably the inner conductor (41) comprises a barrel which has its one end blocked and also which has a cooling medium distribution path formed in its internal space.
In such a construction, heat generated by the plasma generating space (19) can be externally radiated even more rapidly by a cooling medium distributed through the cooling medium distribution path.
Also, according to the invention, preferably the inner conductor (41) houses a barrel (18) disposed therein for supplying a cooling medium, from the interior of which barrel is formed the cooling medium distribution path over the gap between this barrel (18) and the inner conductor (41).
This construction enables effective distribution of the cooling medium, thus making it possible to external radiate the heat generated in the plasma generating space (19) even more rapidly.
Also, according to the invention, preferably an insulator (not shown in FIG. 2) is provided for covering the outside of one end of the inner conductor (41).
In such a construction, the inner conductor (41) is not directly exposed to plasma, thus preventing a contamination (metal reaction product) from being diffused into plasma from the inner conductor (41).
Further, according to the invention, preferably the insulator (16) is cooled by the cooling medium distributed through the cooling medium distribution path.
This arrangement prevents a damage (crack and the like) from occurring on the insulator (16) which damage is caused by thermal stress generated along with plasma.